This report will describe work carried out in this laboratory which resulted in the discovery and characterization of a mammalian mitochondrial transhydrogenase, an enzyme capable of transferring hydrogens from specific hydroxyacids to equally specific ketoacids. This work suggests new catabolic pathways for L-beta-hydroxybutyrate, gamma-hydroxybutyrate and alpha- ketoglutarate and may explain the origin of the elevated levels of certain hydroxyacids found in patients with specific organic acidemias.